It would be helpful to know the model and brand of your outside condenser...just based on the info you have provided, I would say you have a system that has the cheap time/temp defrost method as opposed to the preferred EDD, electronic demand defrost system that reduces/eliminates unnecessary and expensive defrost calls...

IMO

Are you setting back the temperature settings?

tigerdunes, is time/temp defrost a factor when the outside temperature is above freezing? The OP said 40-50 outside.

This booklet shows thermostat settings. How about the settings on the unit itself, what are those, what should they be?

I understand you'd prefer to take care of it yourself but by asking these questions it would seem you don't have enough information or understanding to do that. Some thermostat settings may be overridden by the system settings or vice-versa. If you're not happy with how the system acts, then you're not happy with what the installer did and you should call them back.

Edit to add - It wasn't until I completely replaced the two systems in my house last year that I gained a full appreciation of the mantra "the installer matters more than the equipment". As cloth is to a tailor or clothes designer, as paint is to a painter, the equipment is no more than an element of the many tools and ingredients used by a knowledgeable HVAC contractor to produce a system that functions as desired. Some are much better, some are much worse, than others. No matter which yours is, better to not jump into someone else's work product and tinker.

suburnmd,

no, that's why it's called time/ temp defrost. It has to be cold enough to trigger it.

It doesn't mean that may not be what has happened as controls do fail whether it's time / temp or on demand.

You mean anything can break? Yes, grasshopper. ANYTHING.

Ok let the flaming begin... summer is over and I am feeling a little chilled.

The White-Rogers 1F89-211 is a $48 non-programmable thermostat. So the OP is not using a programmed set back, but could be setting back the temperature manually. But let's assume he sets the thermostat at one temperature only. The questions then becomes how long does the heat pump have to run to maintain the indoor setting on a mild (40-50 degree) day? The OP said the auxiliary heat strips turned on after 10 minutes. A 10 minute run time seems a bit long to maintain the temperature on a mild day. I would investigate why that is happening.

The other question is what algorithm does the thermostat use to decide when to turn on the heat strips? Since the thermostat has no information about the outside temperature, then the algorithm has to be based on either run time duration and/or the delta between current room temperature and temperature setting. I would not expect the algorithm control to be too sophisticated for a $48 product. So I suspect we are observing the algorithm turns on the heat strips after a set run time which the user has no option to change.

I think the OP needs the installer to come back and due a maintenance check. The heat pump may be under performing and running much longer than it should be. The timed defrost may be also compounding the problem, but I will let the heat pump experts debate that one.

It could be a furnace setting that is turning on the auxiliary heat after 10 minutes, whether needed or not.

If you want to control heat strip operation, you may need a different HP thermostat and an outdoor temp sensor. At the outdoor temperatures you mentioned, the heat strip should not be in operation. However, I have seen it happen with a condenser that has time/temp defrost feature. The defrost function and settings should be checked by your installer. Depending on one's location, I always recommend a HP condenser that has electronic demand defrost feature that eliminates/reduces unnecessary defrost calls.

IMO

The OP does not have a furnace.

OP Quote: Some days when it was only 40-50 outside my auxiliary heat was coming on after the unit ran for 10 minutes or so.

My suspicion is that the thermostat is wired for stage 2 heat and it is turning on the auxiliary heat because second stage heat is calling.

Look at your thermostat base and see if there is a jumper between the two points shown in the picture below.

Defrost would (should) not initiate in 40 degree weather and on demand defrost would suffer same fate or worse.

Disclaimer: I am not there, magic 8 ball was summoned for this exercise in forum board magic.

I still think this is the behavior of an inexpensive thermostat with no control as to when the heat strips will turn on. If you put a voltmeter between the W2 connection and ground you should see 24 V appear after 10 minutes.

Also if the OP is observing the flame symbol on the thermostat's display as an indication that the auxiliary heat is turning on, then wouldn't that indicate the thermostat is initiating the call for the heat strips to come on? If it were due to timed defrost, which may also be happening, the thermostat has no feedback from the heat pump that this is happening.

Mike, W2 on this stat is stage 2 heat, the strips would/ should be stage 2. You know when I pull out the magic 8 ball it's a haunch. You have to put things into perspective that:

1. We don't know the heat pump was installed properly. (stranger things have happened right?)

2. We also don't know how it was wired.

3. Timed defrost? It's time & temperature. At 40 degrees defrost will not engage.... unless the thing is so frozen the ice is hitting the temp sensor of the defrost... but again that is not a defrost problem... that is an installation problem. If the unit was that frozen I think the OP would have noticed it.

NOTE: A properly operating heat pump, the condenser coil should not freeze in 40 - 50 degree air temps.